The current method for integrating planar passive devices such as distributive filter elements, inductors, baluns, etc., onto monolithic integrated circuits (ICs) is to fabricate them onto the front-side or active side of the die. Front-side of the die is the side of the substrate having active devices. FIG. 1A illustrates a top view 100 of a monolithic RF (Radio Frequency) IC with planar devices (e.g., Stripline Inter-digital Filter, Inductors, Coupled open circuit lines, and Stub-type Low Pass Filters (LPF)), and with active devices (e.g., Phase Locked Loop (PLL) and input-output transceivers (I/Os)) on the active side of the IC die. The planar devices take up a significant portion of the active region of the die. Making smaller dies is thus challenging.
The industry is also moving towards higher integrated systems. For example, mobile communication devices are manufactured under high demands of cost reduction as well as drive for miniaturization, and therefore the integration of more and more components onto a single System-on-Chip (SoC) is an important focus area. Surface Acoustic Wave (SAW) filters play an important role in modern mobile communication devices and are widely used as Band Pass Filters (BPF) and antenna duplexers. SAW filters may also contain electromechanical components and are fabricated by special micromachining technologies by means of separate devices beneath the IC containing the transmitter and receiver circuits. In multiband communication devices, for example, in Long-Term Evolution (LTE) transceivers, a substantial amount of SAW filters is required which are mostly separate components occupying substantial board area.